This invention relates to the art of screen printing. More particularly, it relates to a method and apparatus for forming an image in a predetermined orientation on a screen to be used for screen printing.
The act of screen printing has come into much greater importance in recent years as the reliance of industry upon mini-circuits became more and more prevalent.
The thick-film technique is one method employed in preparing such mini-circuits. In this method, a refractory substrate called a "chip" is printed with an appropriate paste in the desired pattern and thereafter fired at high temperatures to set the paste and bond it to the substrate. The pattern is formed on the chip by means of a screen printer. In one technique for forming the desired pattern on the screen itself, the entire screen is first coated with a photo-sensitive material. The screen is then covered with an image positive which serves to mask those areas of the screen which will correspond to areas on the chip where no paste is to be applied. The screen and image positive are then exposed to light which results in a reaction taking place only in the unmasked areas of the screen. Subsequently, the screen may then be rinsed or subjected to a reaction whereby only the desired areas of the screen have openings suitable for passing the paste through the screen whereas those areas which where masked by the image positive remain impermeable to such paste.
In the past, the image has been only roughly centered on the screen and the screen held in the screen printing machine by imprecise clamps or screws. Since screens must be periodically discarded and/or cleaned, it is necessary to have numerous screens with the same pattern if a continuous printing operation is desired. Under such circumstances, it has been necessary to adjust the screen printer each time the screen was charged so as to bring the screen image into correct lateral position relative to the substrate holder beneath. A substantial amount of operator time could be saved, and a reduction in the number of chips rejected for quality control reasons could be effected if the image on the screen could be so precisely located with respect to the substrate holder and screen frame that the operator may simply replace one screen for another and continue printing without making any adjustments to the screen printer.